Lithography is widely used in various industrial applications, including the manufacture of integrated circuits, flat panel displays, micro-electro-mechanical systems, micro-optical systems etc. Generally speaking, the lithography process is used for producing a patterned structure. During the manufacture of integrated circuits, a semiconductor wafer undergoes a sequence of lithography-etching steps to produce a plurality of spaced-apart stacks, each formed by a plurality of different layers having different optical properties. Each lithography procedure applied to the wafer results in the pattern on the uppermost layer formed by a plurality of spaced-apart photoresist regions.
To assure the performance of the manufactured products, the applications of the kind specified above require an accurate control of the dimensions of sub-micron features of the obtained pattern When dealing with wafers, the most frequently used dimensions are the layer thickness and the so-called “critical dimension” (CD). CD is the smallest transverse dimension of the developed photoresist, usually the thickness of the finest lines and spaces between these lines. Since the topography of the measured features is rarely an ideal square, additional information found in the height profile, such as slopes, curves etc., may also be valuable in order to improve the control of the fabrication process.
Several Optical CD (OCD) measurement techniques recently developed rely on imaging a certain test pattern in the form of diffraction gratings, which are placed in a special test area of the wafer. The gratings are illuminated by light (typically a laser beam), and the resulting diffraction pattern is analyzed to determine the line width and profile of the gratings. These techniques utilize various methods aimed at amplifying tiny differences in the line-width to obtain macroscopic effects that could be resolved by visible light, although the original differences are more than two orders of magnitude below the wavelength used.
Techniques of the other kind utilize scatterometric measurements, i.e., measurements of the spectral characteristics of a sample. To this end, when dealing with wafers, a test pattern in the form of a grating is placed in the scribe line between the chips. The measurement includes illumination of the grating with a beam of incident light and determining the diffraction efficiency of the grating under various conditions. The diffraction efficiency is a complicated function of a line profile and of the measurement conditions, such as the light wavelength angle of incidence, polarization and diffraction order of collected light, thus providing a wealth of data allowing the extraction of information about the line profile.
Techniques that utilize the principles of scatterometry and are aimed at the characterization of three-dimensional grating structures and determination of line profiles have been disclosed, for example, in the U.S. Pat. Nos. 5,867,276 and 5,963,329. Broadband scatterometry consists of the illumination of a sample with an incident light beam having a broad spectral composition and detecting a beam of light diffracted from the sample with a spectrometer to obtain spectrally-resolved diffraction characteristics of the sample for determining the parameters of the sample.
However, in the above patent, these documents do not describe any specific method of measurements, or the constructional and operational principles of specific adjustment or optimization of numerical aperture for illumination/detection optical systems.